This invention relates to data processing of measurements on free flowing or gas lift oil and gas wells. More particularly, this invention relates to a method which uses type curves for determining the optimum cost-effective production rate for the well as a function of the cost of obtaining particular values of various controllable well parameters, such as production tubing size, wellhead pressure, fracture half-length, gas-lift injection pressure, etc.
The testing of oil and gas wells to measure such things as the size of the fluid reservoir containing the oil and/or gas to be produced, the pressure of the fluid in the reservoir, the porosity of the formations comprising the reservoir, the temperature of the reservoir, etc., and required prerequisite steps to the quantitative determination of the steady state productivity of the well. This measured information, along with a modeling of the various elements which together characterize the well, beginning at the formation up through the production system to the point of sale of the produced oil or gas, has been used in the past to determine the productivity of the well. A commonly used prior-art method to indicate this productivity is through curves which depict the fluid pressure at any given point in the system as a function of the production flow rate past that point. When the indicated pressure is taken at the bottomhole pressure, these curves are called the inflow performance relationship IPR for the reservoir.
When the pseudo-steady state flow regime has been obtained for the well, or for that matter, the intermediate flow condition referred to as the infinite-flow regime, the prior art techniques of testing the well to obtain the IPR curves can be used. Also, simple flow equations have been developed which characterize the infinite-flow and steady state flow conditions from the reservoir. However, neight testing of the well to obtain actual pressure readings or the use of simple flow equations can be used to obtain early time or transient IPR curves for a reservoir. Testing is inadequate because of the transient nature of the IPR curve during early time. Pressure measurements would not be valid at any time other than when taken. Simple flow equations which describe the early time for a reservoir simple do not exist.
Even where simple flow equations are used to describe the performance of the reservoir for the infinite-flow and steady state flow conditions, the equations do not handle the transition period flow conditions when the flow regimes are changing.
When IPR curves for the reservoir have been obtained, it is possible using a technique commonly known in the art as the production systems analysis approach to analyze the well performance. This approach makes it possible to determine the performance of the producing system of the well at any given point in the system by dividing the system into two portions, one portion including everything from the reservoir up to a selected solution point (point of mathematical equivalence) and the other portion including everything from the solution point to the point of sale of the fluid, such as the gas sale line or the stock tank. For each of these two portions, a performance curve of the pressure versus production rate is obtained. These two curves are then plotted on a common graph where the point of intersection of the curves represents the production rate at which the well will produce.
Changes in the performance curves for these two portions of the system may be affected by varying the elements which comprise the producing system or to modify the structure of the formations of the reservoir, such as by fracturing, to effect a change in the production rate. Ideally, it is desirable to always have the point of intersection of the performance curves for the two portions of the system occur as far to the right as possible, i.e., where the production rate is greater and thus greater productivity. Much effort has been expended in the prior art trying to optimize production by moving this intersection point to the right. Such techniques as fracturing of tight formations to increase inflow performance from the reservoir to the production system, reducing the wellhead pressure by putting compressors on the well, selecting different tubing sizes and using gas lift techniques to aid in pumping oil to the surface are just a few commonly used techniques to change the overall performance curves for the components of a well to achieve this greater productivity.
These changes, however, involve an expenditure of money. For example, the cost to hydraulically fracture a tight formation to increase fluid inflow can run into millions of dollars depending on such things as how deep the fracture extends into the reservoir formations. Usually the well operator has little information to determine exactly how much fracturing is needed to obtain the maximum productivity from the well. In other words, the well operator has ways to increase productivity of his well, but has not known quantitatively how much or just what changes to make to the well that will result in the most cost-effective increase in well production.
Accordingly, it would be advantageous to provide a method of obtaining performance curves of the productivity of a well for all flow regimes of the reservoir, including the transient early time performance, as a function of a controllable parameter of the well producing system so that the maximum cost-effective production from the well can be obtained by selecting the proper value according to the expense of obtaining that value.